This is an Xbox 360 Wireless Gaming Reciever, a USB dongle you plug into your PC to connect up to 4 wireless Xbox 360 controllers to your PC, using the XInput protocol. It works great but I ended up with two of them and I only need one as you can connect 4 gamepads to each one. I don't think it's official, but it doesn't seem to impact functionality at all.
eBay